Internal combustion engines are the main source of power for a wide variety of vehicles throughout the world, and include passenger cars, buses, trucks, farm tractors and other equipment, construction equipment, as well as portable equipment such as electrical generators, air compressors and the like. While the engines in most passenger vehicles are powered by gasoline (a.k.a. “petrol”), engines in most commercial vehicles and equipment use conventional diesel fuel. Gasoline has gained popularity because gasoline engines typically produce less harmful emissions, such as soot and unburned hydrocarbons, and are more easily started in colder weather. Diesel engines, which typically use No. 1 or No. 2 diesel fuel, tend to be louder, produce more harmful emissions, and are more difficult to start in cold weather. However, the energy contained in a single gallon of diesel fuel is approximately 25,000 Btus greater than that found in a gallon of gasoline, thus diesel fuel is more efficient.
In recent years, an effort to reduce the use of fossil fuels has prompted widespread use of gasoline/ethanol mixtures, in which approximately 10-20% ethanol is added to gasoline. However, similar hybrid fuel mixtures containing diesel fuel and ethanol have not come into widespread use, despite the disclosure of various methods to form such blends. The ethanol used in gasoline/ethanol blends is typically produced from renewable sources such as by fermentation of plant material, notably corn. It is estimated that about 30% of the transportation fuel used in the United States is diesel fuel. This includes both petroleum-based diesel fuel, and to a much lesser extent, vegetable or animal fat based diesel fuel (“biodiesel”). It is estimated that widespread use of diesel fuel blended with 20% ethanol would result in a decrease in fossil fuel use for transportation purposes of up to 6%.
The demand for fuel ethanol has resulted in an increase in the acreage devoted to the production of grain from which the ethanol is produced. Further gains in production have resulted from increased corn production per acre, and increased ethanol production per kernel. In addition, increases in food production in other nations, such as the Russian Federation, are projected, and could free up starch crops for additional ethanol production. Such factors would allow the blending of ethanol with diesel to become more economically feasible and practical. As the growth in the use of fully electric vehicles, hybrid gasoline burning vehicles, and more fuel efficient gasoline burning vehicles takes place, more ethanol may become available for use in diesel engines. Diesel engines, in contrast, are not as amenable to replacement by electric engines because of their widespread use in heavier vehicles that require more power. Thus a changeover from diesel to electric powered engines is expected to require a significantly longer time, and the demand for economical diesel fuel will continue.
While there has been widespread acceptance of gasoline/ethanol blended fuels, diesel fuel/ethanol blends have achieved limited success. This is due in large part to the technical problem associated with the different physical and chemical properties of ethanol, gasoline and diesel fuel. Such differences account for the different approaches used for the formulation of each of these blended fuels.
Gasoline typically is a lower boiling blend of hydrocarbons, straight and branched chain, as well as unsaturated and aromatic hydrocarbons, which are optimized and blended with additives to provide an octane rating of 97 or higher. Greater unsaturation and aromatic content tends to increase the octane rating.
Diesel fuels are typically higher boiling paraffinic hydrocarbons, substantially free of lower vapor pressure species such as aromatic and unsaturated hydrocarbons, and are optimized for cetane rating of about 40. Cetane rating is a metric used to estimate the performance characteristics of diesel fuel. This number derives from the performance characteristics of a specific mixture of un-branched open-chain alkane molecules (approximately 10 carbons total) that ignite very easily under compression, which are assigned a cetane number of 100; in contrast, alpha-methyl naphthalene (an aromatic hydrocarbon) is assigned a cetane number of 0. In order for diesel fuel to achieve desirable properties, i.e., a desirable cetane number, additives such as alkyl nitrates (principally 2-ethyl hexyl nitrate) are used. A cetane number of 30 or more is required for suitable diesel fuels.
Commercial ethanol is purified by distillation, forming an azeotrope with water (about 95:5 v/v). In the production of fuel ethanol, the bulk of the water is removed using 3 Å molecular sieve drying agents, providing a final water content of 1% v/v max. The cetane number of ethanol is about 8. Fuel ethanol contains 1.96-5.0% denaturant, i.e., a substance is added to the ethanol to make it unsuitable for human consumption. Gasoline itself is a denaturant, and other common denaturing additives include (but are not limited to) materials such as methanol, benzene, toluene, isopropanol, methyl ethyl ketone, methyl isobutyl ketone, pyridine, diethyl phthalate, and naphtha. In some countries, denatured alcohol must be colored blue or purple using an aniline dye, in order to distinguish it from consumption-grade ethanol.
Thus the properties of gasoline and diesel fuel are quite different, each having been optimized to provide properties (i.e., octane and cetane ratings) which are often inversely related, in order to satisfy the requirements of the engines in which they are used. Specifically, a gasoline/ethanol blend and a diesel fuel/ethanol blend, each of which contain the same volume of ethanol and the same volume of contaminating water behave quite differently. It has been reported that anhydrous (dry) alcohol is miscible with diesel fuel, particularly biodiesel in all proportions, at least at ambient temperatures (Blume 1983, 2003). However when operating at reduced temperatures, a diesel/ethanol blend will undergo phase separation at a much higher temperature than a similar gasoline/ethanol blend. In addition, a gasoline/ethanol blend and a diesel fuel/ethanol blend are affected differently by the amount of water present in the blend. For example, a blend consisting of 90% gasoline by volume and 10% anhydrous ethanol by volume can separate at 0° C. if it becomes contaminated with greater than 0.4% by volume of water. A blend consisting of 90% diesel fuel by volume and 10% anhydrous ethanol by volume, however, can separate at 0° C., if it is contaminated with just 0.05% water by volume.
Thus a major drawback in ethanol-diesel fuel blends is that ethanol is immiscible in diesel over a wider range of temperatures and water content, than corresponding ethanol-gasoline blends, resulting in fuel instability due to phase separation (Kwanchareon 2006). Phase separation of ethanol from the diesel/ethanol blend, allowing introduction of ethanol itself into the engine, can cause damage to diesel engines as they are currently designed. It is accepted that attempting to use ethanol itself in a diesel engine is a problem that is “technically very complex . . . ” and requires “ . . . important modifications on the engine hardware in order to overcome the weak auto ignition property of ethanol.” [L. Pidol, et al., Fuel, 85, (5-6), March-April 2006, pp 815-822]
Hence, a more technically feasible solution to the use of ethanol in diesel engines is to provide for a blending agent that prevents such phase separation of diesel/ethanol mixtures at reduced operating temperatures.
Many of the additives and blending agents used to form ethanol/gasoline blends are not particularly useful or compatible with those used to form ethanol/diesel fuel blends. Often this is because the addition of ethanol enhances the octane rating of gasoline, but decreases the cetane number of diesel fuel. “The very thing that makes alcohol an ideal fuel for spark-ignited engines—its high resistance to pinging, due to its high octane rating—works against its easy use in diesel engines.” (Blume, Alcohol can be a gas! Book 4, Chap 25 p. 450). Hansen et al. have published a review of agents used to form ethanol-diesel fuel blends (Bioresource Technology 96, p 277-285 (2005)). Such agents are either miscible directly (co-solvents) and can be “splash blended”, or are emulsifying agents which require heating to achieve blending.
For example, U.S. Pat. Nos. 6,190,427, 6,017,369, 7,311,739, and 7,172,635, describe compositions of diesel fuel and ethanol further comprising a mixture of ethoxylated fatty alcohols and ethanolamides.
U.S. Pat. No. 4,968,320 describes a blend comprising diesel fuel, crude fusel oil, a surfactant and water. The surfactants used are, for example, an alkali metal salt of an alkylbenzenesulfonic acid or of an unsaturated higher fatty acid. The fusel oil may contain some ethanol, in addition to other alcohols.
U.S. Pat. No. 4,451,265 describes a blended fuel comprising a diesel fuel, a lower (C1-C3) alcohol, water and a surfactant system derived from the reaction product of N,N-dimethyl amine and a long-chain fatty acid substance.
McCormick and Parish, “Advanced Petroleum Base Fuels Program and Renewable Diesel Program,” NREL/MP-540-32674, November 2001, review the state of the art for ethanol in diesel fuel (E-Diesel). The report describes the physical properties of 15% ethanol/diesel blends that contain emulsifiers as having lower flash points. This change in property of the fuel, results in a change from a Class II liquid to a Class I liquid.
L. R. Waterland et al, “Safety and Performance Assessment of Ethanol/Diesel Blends (E-Diesel), NREL/SR-540-34817, September 2003, further review the safety and performance aspects of ethanol/diesel blends, citing five additive (emulsifier) vendors: Pure Energy, a blend of alkyl esters of fatty acids, fatty acid alcohols and a polymer; O2 Diesel (formerly AAE Technologies) alkanolamides; Akzo Nobel with a propriety agent called Beraid ED10; Lubrizol and GE Betz, with a phosphite-based additive.
A report (DEH Ethanol Standard 18/2004, International Fuel Quality Center, 2004) describes ethanol/diesel blends as a test fuel in the US, Thailand and Sweden. It refers to a SCANIA additive identified as Etamax D, referenced in US patent application 2000242347, which describes a diesel fuel composition comprising about 60 to about 95% (v/v) ethanol, and up to about 20% (v/v) of a linear dialkyl ether with a chain length of about 10 to about 40, as well as mixtures thereof, and 0 to about 30% (v/v) combustion accelerator. Described combustion accelerators include rapeseed, palm oil or soya oil methyl esters.
Lacking an emulsifying agent (e.g., a surfactant) or a blending agent, the miscibility of ethanol with either diesel fuel or gasoline is essential for its practical use in blended fuel. Miscibility must also be maintained at temperatures where vehicles are normally operated, including from well below freezing (e.g., −15° F.) to as high as 110° F. At low temperatures, waxing or gelling of unblended diesel fuel occurs. Kerosene can be added to solve the problem, but the addition of ethanol generally does not.
Fusel oil or more accurately, crude or unrefined fusel oil is broad term used to describe a byproduct of the fermentation process. According to the Encyclopedia Britannica, it is a mixture of volatile, oily liquids produced in small amounts during alcoholic fermentation. Typically fusel oil contains 60-70 percent amyl alcohols, smaller amounts of n-propyl and isobutyl alcohols, and traces of other components including ethanol. The amount of ethanol may depend on the skill, equipment and techniques of the distiller. Fusel oils alcohols are apparently produced during fermentation from amino acids. Before industrial production of synthetic amyl alcohols began in the 1920s, fusel oil was the only commercial source of these compounds, which are major ingredients of lacquer solvents. In industrial alcohol plants, fusel oil and ethyl alcohol are separated from the fermented liquors via distillation. The fusel oil fraction boils higher than the ethanol fraction. U.S. Pat. No. 4,585,461 further describes fusel oil as a volatile, poisonous mixture of isoamyl, isobutyl, and ethyl alcohols produced as by-products in alcoholic fermentation, of starches, grains, or fruits to produce ethyl alcohol. For example; fusel oil is a by-product in the process of aging wine and beer. As ethanol manufacturers distill the fermented mixture, the high boiling fraction is fusel oil. It is often referred to as the tailings. Fusel oil is foul-smelling and generally considered a nuisance waste material. Proper disposal of fusel oil is required because of its toxic nature, having among its constituents a teratogen and suspected carcinogen. Fusel oils are primarily used as boiler fuel. However, its disadvantages as boiler fuel include (1) 5-18% water content which severely hinders its burning and (2) an oxygen content which reduces its BTU content substantially below gasoline, diesel fuel, or number 4 fuel oil.
The chemical composition of fusel oil is not precisely defined. It depends on such factors as the raw material used for the fermentation (e.g., corn, sugar/molasses, rice, etc.). At least 50 different compounds have been identified in molasses-based fusel oil using gas-liquid chromatography, the major components being 2-methyl-1-butanol and 3-methyl-1-butanol (Karaosmanoglu et al., Energy & Fuels, 1996, 10, 816-820). It was also found to contain about 8.6% water (v/v). It is usually supplied in either a crude or a refined grade. The crude grade is reported (U.S. Pat. No. 4,968,320) to be 38% isoamyl alcohol, 25% isobutyl alcohol, 4.5% isopropyl alcohol, 13% ethanol, 0.5% methanol and 19% water.
The use of a distilled product from fusel oil in gasoline has also been described (Karaosmanoglu 1996): Distillation of the fusel oil was performed to avoid adding additional water to the fuel mixture. Only the portion boiling above 120° C. was used.
A method of manufacturing a cetane improver from fusel oil has been described (U.S. Pat. No. 4,585,461) by mixing the fusel oil with a highly paraffinic hydrocarbon of specific properties, and nitrating the mixture with nitric and sulfuric acid.
To satisfy the increasing demand for cleaner-burning fuel from renewable sources, commercially acceptable, readily available and stable blends of ethanol with diesel fuel would be an attractive solution. To date the complexity of the current methods for making diesel fuel/ethanol blends, together with the economic constraints, have been barriers to the widespread development of such blends. Clearly, more economical and technically feasible solutions are needed in order for diesel fuel/ethanol blends to become as commonplace as gasoline/ethanol blends.